Biaxially oriented poly(ethylene terephthalate) film, which is a typical type of polyester film, is broadly used in various fields, such as industrial material, magnetic recording material, optical material, information material, and packaging material, because of its superior mechanical strength, thermal properties, humidity properties, and other outstanding characteristics.
For packaging material, which particularly requires impact resistance and pinhole resistance, however, biaxially oriented aliphatic polyamide film is often used. Poly(ethylene terephthalate) film is less used because of its hardness, which is reflected by its toughness. For example, in packaging applications particularly requiring gas barrier properties and moisture resistance, it is necessary to have impact resistance and flex pinhole resistance represented by Gelbo test and, in addition, to exhibit superior characteristics in these resistances at low temperature.
Unfortunately, aliphatic polyamide has high hygroscopicity, and consequently produces intrinsic disadvantages, such as deterioration of moisture dimensional stability and flatness. For example, it is difficult to deposit a metal compound for enhancing the gas barrier properties, or printing quality or adhesion with a laminated layer is negatively affected by moisture absorption.
In contrast, polyester has low hygroscopicity and accordingly it does not exhibit the foregoing disadvantages. However, the impact and pinhole resistances, which are required for packaging material, are undesirably inferior, as described above.
In order to enhance the impact resistance and pinhole resistance of polyester to overcome the above-described disadvantages, some approaches have been proposed. For example, Japanese Unexamined Patent Application Publication No. 6-79776 discloses a flexible polyester film having a specific Young's modulus; Japanese Unexamined Patent Application Publication No. 7-330926, a polyester film prepared by blending a poly(tetramethylene glycol) having a specific molecular weight to a polyester in a specific proportion and adding a specific amount of specific particles. Japanese Unexamined Patent Application Publication No. 2001-11213 also discloses a flexible polyester film prepared by adding poly(ethylene terephthalate) to a modified poly(butylene terephthalate) containing a specific amount of poly(tetramethylene glycol). However, these approaches undesirably affect the mechanical characteristics and transparency of the resulting films, or the films onto which aluminium oxide or silicon oxide is deposited exhibit poor transparency. Furthermore, important characteristics, impact resistance and flex pinhole resistance, are insufficient.
Accordingly, it would be advantageous to provide a polyester film having the same characteristics as biaxially oriented polyamide film, such as impact resistance and flex pinhole resistance, and further exhibiting superior gas barrier properties after a metal or metal oxide is deposited on the film, with its inherent characteristics maintained, such as low hygroscopicity, dimensional stability, flatness, and transparency.